During manufacturing, various positions or locations of a workpiece, such as a board or plate, may be required to be accessed or toured for various purposes. Such purposes may include punching or drilling holes at these locations. In some schemes, the workpiece is placed on a movable positioning table, and the tool or processing means is stationary. In other schemes, the workpiece is stationary and the processing means is moved to the various locations of the workpiece. In either scheme, the operating mechanism is generally controlled by an electronic device or a processor unit.
A large number of methods for controlling positioning means exist which differ with regard to their objective.
Most workpieces have a surface which requires numerous distinct positions located thereon to be toured or accessed. One example of such a surface 1 is shown schematically in FIG. 1. The surface 1 includes a plurality of targets arranged thereon. The targets are the individual points or positions 2 which are to be accessed or toured. The location of these targets may be determined, for example, by a rectangular coordinate system X,Y. For functional efficiency, the individual positions 2, which may also be distributed at random, are frequently arranged in a predetermined raster 3. Minimizing the required positioning path normally implies finding a solution to the so-called "Travelling Salesman Problem" (hereinafter referred to as the TSP), i.e., it is required to determine the shortest possible path for accessing each individual position 2. Since accurately computing this problem for a relatively large number of positions (for example, more than 100) generally surpasses the capacity of existing processors, approximation methods have been developed. For minimizing the processing time needed for a workpiece, an essential additional parameter to be included in TSP computations is the acceleration/deceleration characteristic of the positioning mechanism. The positioning paths obtained as a TSP solution depend strongly on the location and the number of predetermined targets. Thus, slight changes of the current pattern arising, for example, from adding or eliminating a few positions, result in a totally different configuration of the respective positioning path. A TSP positioning path 4 is schematically represented in FIG. 2.
In this manner, specific sequences of movements occur for each type of workpiece or variant during the manufacturing process. Consequently, slight (but invariably present) tolerances of the positioning mechanism affect each workpiece differently, i.e. tolerance--related positional deviations vary from one workpiece to another. For manufacturing workpieces for which all the individual positions have to be toured with reproducible accuracy, for instance, in the case of successively produced individual board layers whose plated throughholes have to be accurately aligned during stacking, a pure TSP approach poses precision problems.
The desired accuracy is obtained by abandoning that approach and by positioning each workpiece during the processing cycle according to a predetermined pattern. For this purpose, a predetermined positioning path is travelled irrespective of how the workpieces differ with regard to the location and number of the individual positions to be toured. Examples of this are meander or spiral-shaped paths 5, 6. Such positioning paths are used in engineering and are schematically represented in FIGS. 3 and 4. They allow a very high reproducible accuracy. However, disadvantageously, the very long positioning paths associated with these schemes cause increased time requirements for accessing each position of each workpiece.